The biological technique related to embryo transfer, in-vitro fertilization, transgenesis, cloning, and the like often involves embryo cryopreservation. It is of great concern on how to better freeze embryos so that the embryos are still able to normally develop after they are thawed.
In prior-art methods of freezing embryos, a cryoprotectant is often employed. The cryoprotectant is added into a freezing solution for the purpose of protecting cells against damage from freezing shock. The cryoprotectants, most of which are small molecules of permeability, protect cells against damage from freezing mainly by penetrating into the cells, where hydration occurs so as to enhance viscosity of intracellular fluid. Different cryoprotectants have different effect of cell cryopreservation due to type, concentration, molecular size, permeability, water activity and mechanism of intracellular action. As the concentration of a cryoprotectant in a freezing solution increases, the amount of the cryoprotectant penetrating into cells increases accordingly. Generally, the cryoprotectant is toxic per se, and it will kill the cells when at an excessively high concentration, while will not serve to protect the cells when at an excessively low concentration.
Moreover, There is also data in the prior art showing that the frozen embryos, upon transfer, have a much more reduced birth rate as compared with the fresh embryos. This suggests that freezing or in vitro treatment with a cryoprotectant has influence on the later development of embryos to some degrees.
Therefore, there is a need in the art for a novel solution for freezing embryos, which can improve the freezing resistance and the thawing recovery rate of embryos. Additionally or alternatively, such solution can also reduce the cryoprotectant amount as used, thereby decreasing the toxic side effect on the embryo and achieving effective embryo freezing.